The intent of this work is to extend our knowledge both of the general properties of plasma membrane protein turnover, and of the special properties of the plasma membrane of vascular endothelial cells. Angiotensin-converting enzyme is a cell-surface glycoprotein found on the vascular endothelium, and also found insoluble form in blood. We have observed that cultured endothelial cells from swine aorta not only possess this enzyme in cell-associated form but also release it into the medium in a form suitable for long life in the circultion. Further we have found that radiolabel disappears from cellular converting enzyme in two kinetic components, one with a half-time of 1 hour, and the second, which corresponds to release into the medium, with a half-time of 20 hours. We suggest that the turnover of most plasma membrane proteins is more complex than simple first order decay. We suggest further that the vascular endothelium is the source, in vivo, of a set of soluble blood proteins, which are either secreted by the cells or shed from their surfaces. To test the first hypothesis, we propose to further characterize the rapid turnover component of cellular angiotensin-converting enzyme, in terms of the sub-cellular location, size and carbohydrate composition of the pool; and to determine whether other proteins of the endothelial cell plasma membrane are removed from the cell in two (or more) kinetic components. To test the second, we propose to examine the proteins which are secreted or shed into the growth medium by culturel endothelial cells for immunologic similarity to plasma proteins. If glycoproteins are found which, like converting enzyme, are common to plasma and to the endothelial surface, we will examine whether they have differences in carbohydrate structure analogous to the differences between tissue and plasma convertng enzyme.